Isotope-Based Tracing of Atmospheric Reactive Nitrogen: Progress and Prospects

Over the past several decades, isotopic techniques have advanced from qualitative assessments to quantitative source apportionment of atmospheric reactive nitrogen (Nr), offering unique insights into its sources, formation processes, and environmental impacts. In the review, we synthesize recent developments in sampling and analytical methods, evaluate isotopic end-member values for various Nr sources, and examine fractionation behavior during transformation. We further provide a comparative overview of global patterns and long-term trends in Nr, drawing on ∼1700 isotopic data points from published studies covering major regions including East Asia, North America, and Europe, spanning 1956–2023. We find that isotopic end-member values for many Nr sources are now well constrained, providing a strong basis for source identification. However, wide variability remains for certain sources, such as soil and livestock emissions, indicating the need for further targeted investigation. In addition, isotopic differences are observed among different physical forms of the same compound (e.g., particulate versus aqueous), likely reflecting variations in isotopic fractionation during formation. These effects should therefore be explicitly considered in isotope-based source attribution. The compiled data reveal pronounced regional contrasts in Nr sources and formation pathways. For example, oxidized Nr [e.g., nitrate (NO3–)] in East Asia (δ15N-NO3–: +5.0 ± 5.6‰) shows stronger influence from combustion emissions compared with North America (−0.2 ± 4.9‰) and Europe (−1.0 ± 3.8‰). Interannual variability also differs: values in East Asia peaked around 2017 and have since declined, whereas North America exhibits a nearly opposite trend. Reduced Nr shows comparable regional and temporal variability. These patterns largely reflect differences in energy use, pollution control policies, and the pace of energy transitions. In addition, oxidized Nr formation exhibits a clear latitudinal gradient, with ozone becoming a more dominant oxidant at higher latitudes. This trend underscores differences in atmospheric oxidation capacity and provides observational constraints for previous model predictions. In summary, this review presents current methodologies, isotopic signatures, fractionation processes, and a comprehensive global Nr data set, demonstrating that isotope techniques are now an essential tool for tracing Nr and for advancing understanding and management of the global nitrogen cycle.